Hypopnea (sometimes spelled hypopnœa) is a medical term for a disorder which involves episodes of overly shallow breathing or an abnormally low respiratory rate. This differs from apnea in that there remains some flow of air. Hypopnea events may happen while asleep or while awake.
During sleep, hypopnea is classed as a sleep disorder. With moderate to severe hypopnea, sleep is disturbed such that patients may get a full night's sleep but still not feel rested because they did not get the right kind of sleep. The disruption in breathing causes a drop in blood oxygen level, which may in turn disrupt the stages of sleep.
Daytime hypopnea events are mostly limited to those with severely compromised respiratory muscles, as occurs in some neuromuscular diseases. Similarly, daytime hypopnea can also cause a drop in blood oxygen level.